In the design of aircraft and other vehicles with low radar cross section, the back-scatter from antennas is an important issue. Often the problem is to design antennas that function efficiently over a relatively narrow bandwidth but suppress the back-scatter at frequencies outside this band. At first glance the microstrip patch antenna appears to be an ideal candidate for solving this kind of problem. It is typically thin, making it easy to suppress structural scattering. More importantly, it has a narrow operating bandwidth with an impedance that tends toward a short circuit outside of this band. The problem is that the patch, like other transmission line components, does not resonate at a single frequency. A second resonance typically occurs somewhere between the second and third harmonic, and other resonances follow. At these higher frequencies, antenna back-scatter tends to be large and generally unacceptable.
One solution to this problem is to recess a patch or cavity-backed slot antenna slightly below the surrounding surface. The resulting cavity is filled with a layer of closed cell foam, or some other material with a very low dielectric constant, and then a layer of magnetic radar absorbing material (RAM) is placed on top of the foam. The RAM is brought flush with the surrounding surface, and its edges are usually tapered to provide a gradual transition to the surrounding metallic surface. In the operating band of the antenna the RAM is designed to be somewhat transparent with resulting losses usually not exceeding two or three dB. At higher frequencies the RAM is designed to be much more absorptive so that the antenna, and its back-scatter at higher order resonances, are hidden by the RAM cover material. The use of RAM for back-scatter suppression makes the design relatively large, complex and costly. It is very difficult to obtain a sufficiently sharp frequency cut-off in the RPM to avoid compromising either the radiation efficiency or the back-scatter suppression.
Another approach to the problem is to actually suppress the higher order resonances within the structure of the antenna. a recessed circular patch antenna which suppresses the higher order modes is shown in FIGS. 1 and 2. The antenna includes a high dielectric alumina substrate 11 having a conductive film or layer 12, such as copper, on one surface. The conductive film is etched to form a slot 13. The dielectric substrate 11 is placed in a cavity 14 formed in the support structure 16. The ground plane formed by the recessed supporting structure is electrically connected to the film 17 surrounding the circular patch 18. a coaxial connector 19 is attached to the ground plane with the center conductor 21 extending to the patch 18 and connected to the patch. The position of the connection determines the impedance presented by the antenna. The electric fields across the gap 13 radiate in an omnidirectional pattern into the half space above the ground plane.
The resulting resonance of the patch is determined not simply by the dimensions of the patch but also by the capacitive loading along the edges of the patch. The capacitance of the narrow slot tends to act as a lumped capacitance so that its susceptance monotonically approaches infinity as frequency increases. While this susceptance works well in combination with the susceptance of the patch to form the primary resonance, the larger values of susceptance at higher frequencies tend to short out the higher order resonances. The suppression of higher order resonances by capacitively loading slot edges is smaller, less complex, less costly and more effective than using RAM. However, this approach has required the use of a material with a high dielectric constant to achieve the required value of capacitance. Ceramics such as alumina are suitable for this purpose and are good dielectrics. Typical gap widths on alumina are 0.005 to 0.010 inch, which are quite reasonable. Nevertheless, ceramics are difficult to work with in development, and their dielectric constant varies significantly from lot to lot. Soft substrates with ceramic loading can also be used for this application, but the control of the dielectric constant is even more of a problem. Both materials tend to be relatively costly. What is needed is a way to suppress the higher frequency resonances without using special materials.